A telescopic sight, also referred to as a scope, is a device equipped with a graphic image pattern (referred to as a reticle) that is used to provide an accurate aiming point. The object light from the external scene under observation is combined with the reticle and both the object light and the reticle are presented to the user's eye as a combined image. Telescopic sights are often mounted to guns or used in standalone glassing devices, such as monocular or binocular seeing eyeglasses.
Scopes come in two types—direct view scopes and non-direct view scopes. In a “direct view” scope, the object light that enters the eye is the light that originates from the external scene under observation and passes through the various internal optical elements of the scope to the eye. In contrast, a non-direct view scope is a scope that includes intermediate image capture of external object light (e.g., via a camera) and subsequent re-generation and display of the object image. The original external scene light does not pass through to the user's eye.
FIG. 1 illustrates a conventional direct view scope 100 that includes an objective assembly 105, which faces the object under observation in the external scene, and an ocular assembly 110, which faces the user's eye. Since objective assembly 105 inverts the object image, in order to present an upright object image to the user's eye, direct view scope 100 includes an erector assembly 115 disposed between objective assembly 105 and ocular assembly 110. Erector assembly 115 operates to re-invert the object image.
Direct view scope 100 includes two focal planes: first focal plane 120 and second focal plane 125. These are planes were the object image is brought into focus by the optical assemblies of direct view scope 100. The first focal plane (FFP) 120 is positioned between objective assembly 105 and erector assembly 115 while second focal plane (SFP) 125 is positioned between erector assembly 115 and ocular assembly 110. Since the object image is brought into focus at both of these planes, a reticle can be positioned at either focal plane.
On a variable power telescopic sight, an FFP reticle expands and shrinks along with the object image as magnification is adjusted while on a SFP the reticle remains the same size and shape while the object image appears to grow or shrink. As such, most variable power scopes are SFP. A significant disadvantage of a variable power SFP is that the proportions between the reticle and object target are dependent upon a selected magnification. In other words, angular meaning of the reticle is not constant across the different magnifications. This is a significant drawback for range-finding reticles that have mil-dots or hash markets. Although FFP reticle scopes do not introduce magnification related errors, the same reticle must be used for all magnifications. A reticle that is well suited for high magnification, may not be well suited for low magnification and vice versa.